1. System Preference > Sound
Make sure that "Mute" is not enabled.
2. If you see red light in the headphone port, plug the headphone into and out of the port.
That will flip the microswitch inside the port. Sound may be stuck in the digital mode.
3. Reset PRAM. http://support.apple.com/kb/PH4405
4. Reset SMC. http://support.apple.com/kb/HT3964
Choose the method for:
"Resetting SMC on portables with a battery you should not remove on your own".